Relaxed silicon germanium platform for high speed CMOS electronics and high speed analog circuits

ABSTRACT

Structures and methods for fabricating high speed digital, analog, and combined digital/analog systems using planarized relaxed SiGe as the materials platform. The relaxed SiGe allows for a plethora of strained Si layers that possess enhanced electronic properties. By allowing the MOSFET channel to be either at the surface or buried, one can create high-speed digital and/or analog circuits. The planarization before the device epitaxial layers are deposited ensures a flat surface for state-of-the-art lithography. In accordance with one embodiment of the invention, there is provided a semiconductor structure including a planarized relaxed Si 1−x Ge x  layer on a substrate; and a device heterostructure deposited on said planarized relaxed Si 1−x Ge x  layer including at least one strained layer.

PRIORITY INFORMATION

[0001] This application claims priority from provisional applicationSer. No. 60/273,112 filed Mar. 2, 2001.

BACKGROUND OF THE INVENTION

[0002] The invention relates to the field of relaxed SiGe platforms forhigh speed CMOS electronics and high speed analog circuits.

[0003] Si CMOS as a platform for digital integrated circuits hasprogressed predictably through the industry roadmap. The progress iscreated through device miniaturization, leading to higher performance,greater reliability, and lower cost. However, new bottlenecks in dataflow are appearing as the interconnection hierarchy is expanded.Although digital integrated circuits have progressed at unprecedentedrates, analog circuitry has hardly progressed at all.

[0004] Furthermore, it appears that in the near future, serious economicand technological issues will confront the progress of digitalintegrated circuits.

[0005] The digital and communication chip markets need an enhancement toSi CMOS and the maturing roadmap. One promising candidate material thatimproves digital integrated circuit technology and introduces new analogintegrated circuit possibilities is relaxed SiGe material on Sisubstrates. Relaxed SiGe alloys on Si can have thin layers of Sideposited on them, creating tension in the thin Si layers. Tensile Silayers have many advantageous properties for the basic device inintegrated circuits, the metal-oxide field effect transistor (MOSFET).First, placing Si in tension increases the mobility of electrons movingparallel to the surface of the wafer, thus increasing the frequency ofoperation of the MOSFET and the associated circuit. Second, the bandoffset between the relaxed SiGe and the tensile Si will confineelectrons in the Si layer. Therefore, in an electron channel device(n-channel), the channel can be removed from the surface or ‘buried’.This ability to spatially separate the charge carriers from scatteringcenters such as ionized impurities and the ‘rough’ oxide interfaceenables the production of low noise, high performance analog devices andcircuits.

[0006] A key development in this field was the invention of relaxed SiGebuffers with low threading dislocation densities. The key backgroundinventions in this area are described in U.S. Pat. No. 5,442,205 issuedto Brasen et al. and U.S. Pat. No. 6,107,653 issued to Fitzgerald. Thesepatents define the current best methods of fabricating high qualityrelaxed SiGe.

[0007] Novel device structures in research laboratories have beenfabricated on early, primitive versions of the relaxed buffer. Forexample, strained Si, surface channel nMOSFETs have been created thatshow enhancements of over 60% in intrinsic g_(m) with electron mobilityincreases of over 75% (Rim et al, IEDM 98 Tech. Dig. p. 707). StrainedSi, buried channel devices demonstrating high transconductance and highmobility have also been fabricated (U. Konig, MRS Symposium Proceedings533, 3 (1998)). Unfortunately, these devices possess a variety ofproblems with respect to commercialization. First, the material qualitythat is generally available is insufficient for practical utilization,since the surface of SiGe on Si becomes very rough as the material isrelaxed via dislocation introduction. These dislocations are essentialin the growth of relaxed SiGe layers on Si since they compensate for thestress induced by the lattice mismatch between the materials. For morethan 10 years, researchers have tried to intrinsically control thesurface morphology through epitaxial growth, but since the stress fieldsfrom the misfit dislocations affect the growth front, no intrinsicepitaxial solution is possible. The invention describes a method ofplanarization and regrowth that allows all devices on relaxed SiGe topossess a significantly flatter surface. This reduction in surfaceroughness increases the yield for fine-line lithography, thus enablingthe manufacture of strained Si devices.

[0008] A second problem with the strained Si devices made to date isthat researchers have been concentrating on devices optimized for verydifferent applications. The surface channel devices have been exploredto enhance conventional MOSFET devices, whereas the buried channeldevices have been constructed in ways that mimic the buried channeldevices previously available only in III-V materials systems, likeAlGaAs/GaAs. Recognizing that the Si manufacturing infrastructure needsa materials platform that is compatible with Si, scalable, and capableof being used in the plethora of Si integrated circuit applications, thedisclosed invention provides a platform that allows both the enhancementof circuits based on Si CMOS, as well as the fabrication of analogcircuits. Thus, high performance analog or digital systems can bedesigned with this platform. An additional advantage is that both typesof circuits can be fabricated in the CMOS process, and therefore acombined, integrated digital/analog system can be designed as asingle-chip solution.

[0009] With these advanced SiGe material platforms, it is now possibleto provide a variety of device and circuit topologies that takeadvantage of this new materials system. Exemplary embodiments of theinvention describe structures and methods to fabricate advancedstrained-layer Si devices, and structures and methods to create circuitsbased on a multiplicity of devices, all fabricated from the samestarting material platform. Starting from the same material platform iskey to minimizing cost as well as to allowing as many circuit topologiesto be built on this platform as possible.

SUMMARY OF THE INVENTION

[0010] Accordingly, the invention provides a material platform ofplanarized relaxed SiGe with regrown device layers. The planarizationand regrowth strategy allows device layers to have minimal surfaceroughness as compared to strategies in which device layers are grownwithout planarization. This planarized and regrown platform is a hostfor strained Si devices that can possess optimal characteristics forboth digital and analog circuits. Structures and processes are describedthat allow for the fabrication of high performance digital logic oranalog circuits, but the same structure can be used to host acombination of digital and analog circuits, forming a singlesystem-on-chip.

[0011] In accordance with one embodiment of the invention, there isprovided a semiconductor structure including a planarized relaxedSi_(1−x)Ge_(x) layer on a substrate; and a device heterostructuredeposited on said planarized relaxed Si_(1−x)Ge_(x) layer including atleast one strained layer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a schematic block diagram of a structure including arelaxed SiGe layer epitaxially grown on a Si substrate;

[0013]FIG. 2 is a schematic block diagram of an exemplary structureshowing that the origin of the crosshatch pattern is the stress fieldsfrom injected misfit dislocations;

[0014]FIG. 3 is a table showing surface roughness data for relaxed SiGebuffers produced by dislocation injection via graded SiGe layers on Sisubstrates;

[0015] FIGS. 4A-4D show an exemplary process flow and resulting platformstructure in accordance with the invention;

[0016] FIGS. 5A-5D are schematic diagrams of the corresponding processflow and layer structure for a surface channel FET platform inaccordance with the invention;

[0017] FIGS. 6A-6D are schematic diagrams of the corresponding processflow and layer structure for a buried channel FET platform in accordancewith the invention;

[0018] FIGS. 7A-7D are schematic diagrams of a process flow for asurface channel MOSFET in accordance with the invention;

[0019]FIGS. 8A and 8B are schematic block diagrams of surface channeldevices with protective layers;

[0020]FIGS. 9A and 9B are schematic block diagrams of surface channeldevices with Si layers on Ge-rich layers for use in silicide formation;

[0021] FIGS. 10 is schematic diagram of a buried channel MOSFET afterdevice isolation in accordance with the invention;

[0022]FIG. 11 is a schematic flow of the process, for anyheterostructure FET device deposited on relaxed SiGe, in accordance withthe invention;

[0023] FIGS. 12A-12D are schematic diagrams of a process flow in thecase of forming the surface channel MOSFET in the top strained Si layerin accordance with the invention;

[0024] FIGS. 13A-13D are schematic diagrams of a process flow in thecase of forming the surface channel MOSFET in the buried strained Silayer in accordance with the invention; and

[0025]FIGS. 14A and 14B are schematic diagrams of surface and buriedchannel devices with Si_(1−y)Ge_(y) channels on a relaxed Si_(1−z)Ge_(z)layer.

DETAILED DESCRIPTION OF THE INVENTION

[0026]FIG. 1 is a schematic block diagram of a structure 100 including arelaxed SiGe layer epitaxially grown on a Si substrate 102. In thisstructure, a compositionally graded buffer layer 104 is used toaccommodate the lattice mismatch between the uniform SiGe layer 106 andthe Si substrate. By spreading the lattice mismatch over a distance, thegraded buffer minimizes the number of dislocations reaching the surfaceand thus provides a method for growing high-quality relaxed SiGe filmson Si.

[0027] Any method of growing a high-quality, relaxed SiGe layer on Siwill produce roughness on the surface of the SiGe layer in a well-knowncrosshatch pattern. This crosshatch pattern is typically a few hundredangstroms thickness over distances of microns. Thus, the crosshatchpattern is a mild, undulating surface morphology with respect to thesize of the electron or hole. For that reason, it is possible to createindividual devices that achieve enhancements over their control Sidevice counterparts. However, commercialization of these devicesrequires injection of the material into the Si CMOS process environmentto achieve low cost, high performance targets. This processingenvironment requires that the material and device characteristics haveminimal impact on the manufacturing process. The crosshatch pattern onthe surface of the wafer is one limiting characteristic of relaxed SiGeon Si that affects the yield and the ease of manufacture. Greaterplanarity is desired for high yield and ease in lithography.

[0028] The origin of the crosshatch pattern is the stress fields fromthe injected misfit dislocations. This effect is depicted by theexemplary structure 200 shown in FIG. 2. By definition, the dislocationsmust be introduced in order to accommodate the lattice-mismatch betweenthe SiGe alloy and the Si substrate. The stress fields originate at thedislocations, and are terminated at the surface of the film. However,the termination at the surface creates crystal lattices that vary fromplace to place on the surface of the wafer. Since growth rate can becorrelated to lattice constant size, different thicknesses of depositionoccur at different points on the wafer. One may think that thick layergrowth beyond the misfit dislocations will smooth the layer of thesethickness differences. Unfortunately, the undulations on the surfacehave a relatively long wavelength; therefore, surface diffusion istypically not great enough to remove the morphology.

[0029]FIG. 3 is a table that displays surface roughness data for relaxedSiGe buffers produced by dislocation injection via graded SiGe layers onSi substrates. Note that the as-grown crosshatch pattern for relaxedSi_(0.8)Ge_(0.2) buffers creates a typical roughness of approximately7.9 nm. This average roughness increases as the Ge content in therelaxed buffer is increased. Thus, for any SiGe layer that is relaxedthrough dislocation introduction during growth, the surface roughness isunacceptable for state-of-the-art fabrication facilities. After theprocess in which the relaxed SiGe is planarized, the average roughnessis less than 2 nm (typically 0.57 nm), and after device layerdeposition, the average roughness is 0.77 nm with a 1.5 μm regrowththickness. Therefore, after the complete structure is fabricated, overone order of magnitude of roughness reduction can be achieved.

[0030] The regrowth device layers can be either greater than or lessthan the critical thickness of the regrowth layer. In general, in anylattice-mismatched epitaxial growth, thin layers can be depositedwithout fear of dislocation introduction at the interface. At a greatenough thickness, any lattice-mismatch between the film and substratewill introduce misfit dislocations into the regrown heterostructure.These new dislocations can cause additional surface roughness. Thus, ifthe lattice-mismatch between the regrowth device layers and relaxed SiGebuffer is too great, the effort of planarizing the relaxed SiGe may belost since massive dislocation introduction will roughen the surface.

[0031] There are two distinct possibilities with respect to the regrowththickness and the quality of surface. If the regrowth layers are verythin, then exact lattice matching of the regrowth layer composition andthe relaxed buffer composition is not necessary. In this case, thesurface roughness will be very low, approximately equal to thepost-planarization flatness. However, in many applications for devices,the regrowth layer thickness will be 1-2 μm or more. For a 1% differencein Ge concentration between the relaxed SiGe and the regrowth layer, thecritical thickness is approximately 0.5 μm. Thus, if optimal flatness isdesired, it is best to keep the regrowth layer below approximately 0.5μm unless excellent control of the uniformity of Ge concentration acrossthe wafer is achieved. Although this composition matching is achievablein state-of-the-art tools, FIG. 3 shows that less precise matching,i.e., within 2% Ge, results in misfit dislocation introduction andintroduction of a new crosshatch pattern. However, because the latticemismatch is so small, the average roughness is still very low,approximately 0.77 nm. Thus, either lattice-matching or slight mismatchwill result in excellent device layer surfaces for processing.

[0032] It is also noted that the relaxed SiGe alloy with surfaceroughness may not necessarily be a uniform composition relaxed SiGelayer on a graded composition layer. Although this material layerstructure has been shown to be an early example of high quality relaxedSiGe, there are some disadvantages to this structure. For example, SiGealloys possess a much worse coefficient of thermal conductivity thanpure Si. Thus, for electronic devices located at the surface, it may berelatively difficult to guide the heat away from the device areas due tothe thick graded composition layer and uniform composition layer.

[0033] Another exemplary embodiment of the invention, shown in FIGS.4A-4D, solves this problem and creates a platform for high power SiGedevices. FIGS. 4A-4D show an exemplary process flow and resultingplatform structure in accordance with the invention. The structure isproduced by first forming a relaxed uniform SiGe alloy 400 via acompositionally graded layer 402 on a Si substrate 404. The SiGe layer400 is then transferred to a second Si substrate 406 using conventionalbonding. For example, the uniform SiGe alloy 400 on the graded layer 402can be planarized to remove the crosshatch pattern, and that relaxedSiGe alloy can be bonded to the Si wafer. The graded layer 402 and theoriginal substrate 404 can be removed by a variety of conventionalprocesses. For example, one process is to grind the original Sisubstrate away and selectively etch to the SiGe, either by a controlleddry or wet etch, or by embedding an etch stop layer. The end result is arelaxed SiGe alloy 400 on Si without the thick graded layer. Thisstructure is more suited for high power applications since the heat canbe conducted away from the SiGe layer more efficiently.

[0034] The bond and substrate removal technique can also be used toproduce SiGe on insulator substrates, or SGOI. An SGOI wafer is producedusing the same technique shown in FIGS. 4A-4D; however, the secondsubstrate is coated with a SiO₂ layer before bonding. In an alternativeembodiment, both wafers can be coated with SiO₂ to enable oxide-to-oxidebonding. The resulting structure after substrate removal is a highquality, relaxed SiGe layer on an insulating film. Devices built on thisplatform can utilize the performance enhancements of both strained Siand the SOI architecture.

[0035] It will be appreciated that in the scenario where the SiGe layeris transferred to another host substrate, one may still need toplanarize before regrowing the device layer structure. The SiGe surfacecan be too rough for state of the art processing due to the substrateremoval technique. In this case, the relaxed SiGe is planarized, and thedevice layers are regrown on top of the high-quality relaxed SiGesurface.

[0036] Planarization of the surface via mechanical or other physicalmethods is required to flatten the surface and to achieve CMOS-qualitydevices. However, the field effect transistors (FETs) that allow forenhanced digital and analog circuits are very thin, and thus would beremoved by the planarization step. Thus, a first part of the inventionis to realize that relaxed SiGe growth and planarization, followed bydevice layer regrowth, is key to creating a high-performance, high yieldenhanced CMOS platform. FIGS. 5 and 6 show the process sequence andregrowth layers required to create embodiments of surface channel andburied channel FETs, respectively.

[0037] FIGS. 5A-5D are schematic diagrams of a process flow andresulting layer structure in accordance with the invention. FIG. 5Ashows the surface roughness 500, which is typical of a relaxed SiGealloy 502 on a substrate 504, as an exaggerated wavy surface. Note thatthe substrate is labeled in a generic way, since the substrate coulditself be Si, a relaxed compositionally graded SiGe layer on Si, oranother material in which the relaxed SiGe has been transferred througha wafer bonding and removal technique. The relaxed SiGe alloy 502 isplanarized (FIG. 5B) to remove the substantial roughness, and thendevice regrowth layers 506 are epitaxially deposited (FIG. 5C). It isdesirable to lattice-match the composition of the regrowth layer 506 asclosely as possible to the relaxed SiGe 502; however, a small amount ofmismatch and dislocation introduction at the interface is tolerablesince the surface remains substantially planar. For a surface channeldevice, a strained Si layer 508 of thickness less than 0.1 μm is thengrown on top of the relaxed SiGe 502 with an optional sacrificial layer510, as shown in FIG. 5D. The strained layer 508 is the layer that willbe used as the channel in the final CMOS devices.

[0038] FIGS. 6A-6D are schematic diagrams of the corresponding processflow and layer structure for a buried channel FET platform in accordancewith the invention. In this structure, the regrowth layers 606 include alattice matched SiGe layer 602, a strained Si channel layer 608 with athickness of less than 0.05 μm, a SiGe separation or spacer layer 612, aSi gate oxidation layer 614, and an optional sacrificial layer 610 usedto protect the heterostructure during the initial device processingsteps.

[0039] Once the device structure has been deposited, the rest of theprocess flow for device fabrication is very similar to that of bulk Si.A simplified version of the process flow for a surface channel MOSFET inaccordance with the invention is shown in FIGS. 7A-7D. This surfacechannel MOSFET contains a relaxed SiGe layer 700 and a strained Si layer702. The device isolation oxide 704, depicted in FIG. 7A, is typicallyformed first. In this step, the SiN layer 706, which is on top of a thinpad oxide layer 708, serves as a hard mask for either local oxidation ofsilicon (LOCOS) or shallow trench isolation (STI). Both techniques use athick oxide (relative to device dimensions) to provide a high thresholdvoltage between devices; however, STI is better suited forsub-quarter-micron technologies. FIG. 7B is a schematic of the devicearea after the gate oxide 716 growth and the shallow-source drainimplant. The implant regions 710 are self-aligned by using a poly-Sigate 712 patterned with photoresist 714 as a masking layer.Subsequently, deep source-drain implants 718 are positioned usingconventional spacer 720 formation and the device is electricallycontacted through the formation of silicide 722 at the gate andsilicide/germanides 724 at the source and drain (FIG. 7C). FIG. 7D is aschematic of the device after the first level of metal interconnects 726have been deposited and etched.

[0040] Since there are limited-thickness layers on top of the entirestructure, the removal of surface material during processing becomesmore critical than with standard Si. For surface channel devices, thestructure that is regrown consists primarily of nearly lattice-matchedSiGe, and a thin surface layer of strained Si. Many of the processesthat are at the beginning of a Si fabrication sequence strip Si from thesurface. If the processing is not carefully controlled, the entirestrained Si layer can be removed before the gate oxidation. Theresulting device will be a relaxed SiGe channel FET and thus thebenefits of a strained Si channel will not be realized.

[0041] A logical solution to combat Si removal during initial processingis to make the strained Si layer thick enough to compensate for thisremoval. However, thick Si layers are not possible for two reasons.First, the enhanced electrical properties originate from the fact thatthe Si is strained and thick layers experience strain relief through theintroduction of misfit dislocations. Second, the misfit dislocationsthemselves are undesirable in significant quantity, since they canscatter carriers and increase leakage currents injunctions.

[0042] In order to prevent removal of strained Si layers at the surface,the cleaning procedures before gate oxidation must be minimized and/orprotective layers must be applied. Protective layers are useful sincetheir removal can be carefully controlled. Some examples of protectivelayers for surface channel devices are shown in FIGS. 8A and 8B. FIG. 8Ashows a strained Si heterostructure of a relaxed SiGe layer 800 and astrained Si channel layer 802 protected by a surface layer 804 of SiGe.The surface SiGe layer 804 should have a Ge concentration similar tothat of the relaxed SiGe layer 800 below, so that the thickness is notlimited by critical thickness constraints. During the initial cleans,the SiGe sacrificial layer is removed instead of the strained Si channellayer. The thickness of the sacrificial layer can either be tuned toequal the removal thickness, or can be made greater than the removalthickness. In the latter case, the excess SiGe can be selectivelyremoved before the gate oxidation step to reveal a clean, strained Silayer at the as grown thickness. If the particular fabrication facilityprefers a Si terminated surface, a sacrificial Si layer may be depositedon top of the SiGe sacrificial cap layer.

[0043]FIG. 8B shows a structure where a layer 806 of SiO₂ and a surfacelayer 808 of either a poly-crystalline or an amorphous material are usedas protective layers. In this method, an oxide layer is either grown ordeposited after the epitaxial growth of the strained Si layer.Subsequently, a polycrystalline or amorphous layer of Si, SiGe, or Ge isdeposited. These semiconductor layers protect the strained-Si layer inthe same manner as a SiGe cap during the processing steps before gateoxidation. Prior to gate oxidation, the poly/amorphous and oxide layersare selectively removed. Although the sacrificial layers are shown asprotection for a surface channel device, the same techniques can beemployed in a buried channel heterostructure.

[0044] Another way in which conventional Si processing is modified isduring the source-drain silicide-germanide formation (FIG. 7C). Inconventional Si processing, a metal (typically Ti, Co, or Ni) is reactedwith the Si and, through standard annealing sequences, low resistivitysuicides are formed. However, in this case, the metal reacts with bothSi and Ge simultaneously. Since the silicides have much lower freeenergy than the germanides, there is a tendency to form a silicide whilethe Ge is expelled. The expelled germanium creates agglomeration andincreases the resistance of the contacts. This increase in seriesresistance offsets the benefits of the extra drive current from theheterostructure, and negates the advantages of the structure.

[0045] Ti and Ni can form phases in which the Ge is not rejectedseverely, thus allowing the formation of a good contact. Co is much moreproblematic. However, as discussed above for the problem of Si removal,a protective layer(s) at the device epitaxy stage can be applied insteadof optimizing the SiGe-metal reaction. For example, the strained Si thatwill become the surface channel can be coated with a high-Ge-contentSiGe alloy (higher Ge content than the initial relaxed SiGe), followedby strained Si. Two approaches are possible using these surface contactlayers. Both methods introduce thick Si at the surface and allow theconventional silicide technology to be practiced without encounteringthe problems with SiGe-metal reactions.

[0046] The first approach, shown on a surface channel heterostructure900 in FIG. 9A, uses a Ge-rich layer 906 thin enough that it issubstantially strained. The layer 906 is provided on a strained Sichannel layer 904 and relaxed SiGe layer 902. In this case, if asubsequent Si layer 908 is beyond the critical thickness, thecompressive Ge-rich layer 906 acts as a barrier to dislocations enteringthe strained Si channel 904. This barrier is beneficial sincedislocations do not adversely affect the silicide process; thus, theirpresence in the subsequent Si layer 908 is of no consequence. However,if the dislocations were to penetrate to the channel, there would beadverse effects on the device.

[0047] A second approach, shown in FIG. 9B, is to allow a Ge-rich layer910 to intentionally exceed the critical thickness, thereby causingsubstantial relaxation in the Ge-rich layer. In this scenario, anarbitrarily thick Si layer 912 can be applied on top of the relaxedGe-rich layer. This layer will contain more defects than the strainedchannel, but the defects play no role in device operation since this Siis relevant only in the silicide reaction. In both cases, the process isfree from the metal-SiGe reaction concerns, since the metal will reactwith Si-only. Once the silicide contacts have been formed, the rest ofthe sequence is a standard Si CMOS process flow, except that the thermalbudget is carefully monitored since, for example, thesilicide-germanicide (if that option is used) typically cannot tolerateas high a temperature as the conventional silicide. A major advantage ofusing Si/SiGe FET heterostructures to achieve enhanced performance isthe compatibility with conventional Si techniques. Many of the processesare identical to Si CMOS processing, and once the front-end of theprocess, i.e., the processing of the Si/SiGe heterostructure, iscomplete, the entire back-end process is uninfluenced by the fact thatSi/SiGe lies below.

[0048] Even though the starting heterostructure for the buried channeldevice is different from that of the surface channel device, its processflow is very similar to the surface channel flow shown in FIGS. 7A-7D.FIG. 10 is a schematic block diagram of a buried channel MOSFETstructure 1000 after the device isolation oxide 1016 has been formedusing a SiN mask 1014. In this case, the strained channel 1002 on afirst SiGe layer 1010 is separated from the surface by the growth ofanother SiGe layer 1004, followed by another Si layer 1006. This Silayer is needed for the gate oxide 1008 since gate-oxide formation onSiGe produces a very high interface state density, thus creatingnon-ideal MOSFETs. One consequence of this Si layer, is that if it istoo thick, a substantial portion of the Si layer will remain after thegate oxidation. Carriers can populate this residual Si layer, creating asurface channel in parallel with the desired buried channel and leadingto deleterious device properties. Thus, the surface layer Si must bekept as thin as possible, typically less than 50Å and ideally in therange of 5-15 Å.

[0049] Another added feature that is necessary for a buried channeldevice is the supply layer implant. The field experienced in thevertical direction when the device is turned on is strong enough to pullcarriers from the buried channel 1002 and force them to populate a Sichannel 1006 near the Si/SiO₂ interface 1012, thus destroying anyadvantage of the buried channel. Thus, a supply layer of dopant must beintroduced either in the layer 1004 between the buried channel and thetop Si layer 1006, or below the buried channel in the underlying SiGe1010. In this way, the device is forced on with little or no appliedvoltage, and turned off by applying a voltage (depletion mode device).

[0050]FIG. 11 is a schematic flow of the process, for anyheterostructure FET device deposited on relaxed SiGe, in accordance withthe invention. The main process steps are shown in the boxes, andoptional steps or comments are shown in the circles. The first threesteps (1100,1102,1104) describe the fabrication of the strained siliconheterostructure. The sequence includes production of relaxed SiGe on Si,planarization of the SiGe, and regrowth of the device layers. Once thestrained heterostructure is complete (1106), MOS fabrication begins withdevice isolation (1112) using either STI (1110) or LOCOS (1108). Beforeproceeding to the gate oxidation, buried channel devices undergo asupply and threshold implant (1114), and any protective layers appliedto either a buried or surface channel heterostructure must beselectively removed (1116). The processing sequence after the gateoxidation (1118) is similar to conventional Si CMOS processing. Thesesteps include gate deposition, doping, and definition (1120),self-aligned shallow source-drain implant (1122), spacer formation(1124), self-aligned deep source-drain implant (1126), salicideformation (1128), and pad isolation via metal deposition and etch(1130). The steps requiring significant alteration have been discussed.

[0051] One particular advantage of the process of FIG. 11 is that itenables the use of surface channel and buried channel devices on thesame platform. Consider FIGS. 12A-12D and FIGS. 13A-13D, which show auniversal substrate layer configuration and a process that leads to theco-habitation of surface and buried channel MOSFETs on the same chip.The universal substrate is one in which both surface channel and buriedchannel devices can be fabricated. There are two possibilities infabricating the surface channel device in this sequence, shown in FIGS.12 and 13. The process flows for combining surface and buried channelare similar to the previous process described in FIG. 7. Therefore, onlythe critical steps involved in exposing the proper gate areas are shownin FIGS. 12 and 13.

[0052]FIGS. 12A and 13A depict the same basic heterostructure 1200,1300for integrating surface channel and buried channel devices. There is asurface strained Si layer 1202,1302, a SiGe spacer layer 1204,1304, aburied strained Si layer 1206,1306, and a relaxed platform of SiGe1208,1308. Two strained Si layers are necessary because the buriedchannel MOSFET requires a surface Si layer to form the gate oxide and aburied Si layer to form the device channel. The figures also show adevice isolation region 1210 that separates the buried channel devicearea 1212,1312 from the surface channel device area 1214,1314.

[0053] Unlike the buried channel device, a surface channel MOSFET onlyrequires one strained Si layer. As a result, the surface channel MOSFETcan be fabricated either in the top strained Si layer, as shown in FIGS.12B-12D, or the buried Si layer channel, as shown in FIGS. 13B-13D. FIG.12B is a schematic diagram of a surface channel gate oxidation 1216 inthe top Si layer 1202. In this scenario, a thicker top Si layer isdesired, since after oxidation, a residual strained Si layer must bepresent to form the channel. FIG. 12B also shows a possible position forthe buried channel supply implant 1218, which is usually implantedbefore the buried channel gate oxide is grown. Since the top Si layer isoptimized for the surface channel device, it may be necessary to stripsome of the top strained Si in the regions 1220 where buried channeldevices are being created, as shown in FIG. 12C. This removal isnecessary in order to minimize the surface Si thickness after gate oxide1222 formation (FIG. 12D), and thus avoid the formation of a paralleldevice channel.

[0054] When a surface channel MOSFET is formed in the buried strained Silayer, the top strained Si layer can be thin, i.e., designed optimallyfor the buried channel MOSFET. In FIG. 13B, the top strained Si and SiGelayers are removed in the region 1312 where the surface channel MOSFETsare formed. Because Si and SiGe have different properties, a range ofselective removal techniques can be used, such as wet or dry chemicaletching. Selective oxidation can also be used since SiGe oxidizes atmuch higher rates than Si, especially under wet oxidation conditions.FIG. 13C shows the gate oxidation 1314 of the surface channel device aswell as the supply layer implant 1316 for the buried channel device.Finally, FIG. 13D shows the position of the buried channel gate oxide1318. No thinning of the top Si layer is required prior to the oxidationsince the epitaxial thickness is optimized for the buried channeldevice. Subsequent to these initial steps, the processing for eachdevice proceeds as previously described.

[0055] Another key step in the process is the use of a localized implantto create the supply layer needed in the buried channel device. In aMOSFET structure, when the channel is turned on, large vertical fieldsare present that bring carriers to the surface. The band offset betweenthe Si and SiGe that confines the electrons in the buried strained Silayer is not large enough to prevent carriers from being pulled out ofthe buried channel. Thus, at first, the buried channel MOSFET wouldappear useless. However, if enough charge were present in the top SiGelayer, the MOSFET would become a depletion-mode device, i.e. normally onand requiring bias to turn off the channel. In the surface/buriedchannel device platform, a supply layer implant can be created in theregions where the buried channel will be fabricated, thus easing processintegration. If for some reason the supply layer implant is notpossible, note that the process shown in FIG. 11 in which the surfacechannel is created on the buried Si layer is an acceptable process,since the dopant can be introduced into the top SiGe layer duringepitaxial growth. The supply layer is then removed from the surfacechannel MOSFET areas when the top SiGe and strained Si layers areselectively etched away.

[0056] In the processes described in FIGS. 10, 12 and 13, it is assumedthat the desire is to fabricate a buried channel MOSFET. If the oxide ofthe buried channel device is removed, one can form a buried channeldevice with a metal gate (termed a MODFET or HEMT). The advantage ofthis device is that the transconductance can be much higher since thereis a decrease in capacitance due to the missing oxide. However, thereare two disadvantages to using this device. First, all thermal processesafter gate definition have to be extremely low temperature, otherwisethe metal will react with the semiconductor, forming an alloyed gatewith a very low, or non-existent, barrier. Related to this issue is thesecond disadvantage. Due to the low thermal budget, the source and drainformation and contacts are typically done before the gate definition.Inverting these steps prevents the gate from being self-aligned to thesource and drain, thus increasing the series resistance between the gateand the source and drain. Therefore, with a carefully designed buriedchannel MOSFET, the self-aligned nature can be a great advantage indevice performance. Another benefit of the MOSFET structure is that thegate leakage is very low.

[0057] The combination of buried n-channel structures with n and p typesurface channel MOSFETs has been emphasized heretofore. It is importantto also emphasize that in buried n-channel devices as well as in surfacechannel devices, the channels need not be pure Si. Si_(1−y)Ge_(y)channels can be used to increase the stability during processing. FIGS.14A and 14B are schematic diagrams of surface 1400 and buried 1450channel devices with Si_(1−y)Ge_(y) channels 1402 on a relaxedSi_(1−z)Ge_(z) layer 1404. The devices are shown after salicidation andthus contain a poly-Si gate 1410, gate oxide 1408, suicide regions 1412,spacers 1414, and doped regions 1416. In the surface channel device1400, a thin layer 1406 of Si must be deposited onto the Si_(1−y)Ge_(y)layer 1402 to form the gate oxide 1408, as previously described forburied channel devices. In the buried Si_(1−y)Ge_(y) channel device1450, the device layer sequence is unchanged and consists of a buriedstrained channel 1402, a SiGe spacer layer 1418, and a surface Si layer1420 for oxidation.

[0058] To maintain tensile strain in the channel of an NMOS device, thelattice constant of the channel layer must be less than that of therelaxed SiGe layer, i.e., y must be less than z. Since n-channel devicesare sensitive to alloy scattering, the highest mobilities result whenthe Ge concentration in the channel is low. In order to have strain onthis channel layer at a reasonable critical thickness, the underlyingSiGe should have a Ge concentration in the range of 10-50%.

[0059] Experimental data indicates that p channels are less sensitive toalloy scattering. Thus, surface MOSFETs with alloy channels are alsopossible. In addition, the buried channel devices can be p-channeldevices simply by having the Ge concentration in the channel, y, greaterthan the Ge concentration in the relaxed SiGe alloy, z, and by switchingthe supply dopant from n-type to p-type. This configuration can be usedto form Ge channel devices when=1 and 0.5<z<0.9.

[0060] With the ability to mix enhancement mode surface channel devices(n and p channel, through implants as in typical Si CMOS technology) anddepletion-mode buried channel MOSFETs and MODFETs, it is possible tocreate highly integrated digital/analog systems. The enhancement modedevices can be fabricated into high performance CMOS, and the regions ofan analog circuit requiring the high performance low-noise depletionmode device can be fabricated in the buried channel regions. Thus, it ispossible to construct optimal communication stages, digital processingstages, etc. on a single platform. These different regions are connectedelectrically in the backend of the Si CMOS chip, just as transistors areconnected by the back-end technology today. Thus, the only changes tothe CMOS process are some parameters in the processes in the fabricationfacility, and the new material, but otherwise, the entire manufacturingprocess is transparent to the change. Thus, the economics favor such aplatform for integrated Si CMOS systems on chip.

[0061] Although the present invention has been shown and described withrespect to several preferred embodiments thereof, various changes,omissions and additions to the form and detail thereof, may be madetherein, without departing from the spirit and scope of the invention.

What is claimed is:
 1. A semiconductor structure comprising: aplanarized relaxed Si_(1−x)Ge_(x) layer on a substrate; and a deviceheterostructure deposited on said planarized relaxed Si_(1−x)Ge_(x)layer including at least one strained layer.
 2. The structure of claim1, wherein said strained layer comprises Si_(1−y)Ge_(y) with y<x.
 3. Thestructure of claim 1, wherein said strained layer comprisesSi_(1−y)Ge_(y) with y>x.
 4. The structure of claim 1, wherein the deviceheterostructure comprises a Si_(1−z)Ge_(z) layer in which z isapproximately equal to x; a Si_(1−y)Ge_(y) layer with y<x; and a layerof Si.
 5. The structure of claim 1, wherein the device heterostructurecomprises a Si_(1−z)Ge_(z) layer in which z is approximately equal to x;a Si_(1−y)Ge_(y) layer with y>x; and a layer of Si.
 6. The structure ofclaim 1, wherein the device heterostructure comprises a Si_(1−z)Ge_(z)layer in which z is approximately equal to x; and a layer of Si.
 7. Thestructure of claim 5, wherein y is approximately
 1. 8. The structure ofclaim 6, wherein both x and z are greater than 0.1 and less than orequal to 0.5
 9. The structure of claim 8, wherein the layer of Si isless than 0.1 μm.
 10. The structure of claim 7, wherein both x and z aregreater than 0.5 and less than or equal to 0.9.
 11. The structure ofclaim 10, wherein the layer of Si is less than 0.005 μm.
 12. Thestructure of claim 1, wherein the device heterostructure comprises aSi_(1−z)Ge_(z) layer in which z is approximately equal to x; a secondlayer of Si_(1−y)Ge_(y) with y<x; a third Si_(1−w)Ge_(w) layer in whichw is approximately x; and a layer of Si.
 13. The structure of claim 12,wherein y is approximately
 0. 14. The structure of claim 13, wherein0.1<×<0.5 and the thickness of the second Si_(1−y)Ge_(y) layer is lessthan 0.05μm.
 15. The structure of claim 14, wherein the layer of Si isless than 0.005 μm.
 16. The structure of claim 1, wherein the deviceheterostructure comprises a Si_(1−z)Ge_(z) layer in which z isapproximately equal to x; a second layer of Si_(1−y)Ge_(y) layer withy>x; a third Si_(1−w)Ge_(w) layer in which w is approximately x; and alayer of Si.
 17. The structure of claim 16, wherein y isapproximately
 1. 18. The structure of claim 17, wherein 0.5<×<0.9 andthe thickness of the second Si_(1−y)Ge_(y) layer is less than 0.05 μm.19. The structure of claim 18, wherein the layer of Si is less than0.005 μm.
 20. The structure of claim 1, wherein the substrate comprisesrelaxed graded composition SiGe layers on Si.
 21. The structure of claim1, wherein the substrate comprises Si.
 22. The structure of claim 21,wherein the relaxed SiGe/Si structure is formed through wafer bonding.23. The structure of claim 1, wherein the substrate comprises Si with alayer of SiO₂.
 24. The structure of claim 23, wherein the relaxedSiGe/SiO₂/Si structure is formed through wafer bonding.